The present invention relates to the joining of two workpieces by a welding apparatus and technique. More specifically, the present invention relates to joining two workpieces via homopolar generator welding.
Homopolar pulse welding (HPW) utilizes a high current, low voltage pulse produced by a homopolar generator to rapidly resistance heat an interface between two components to forging temperature. In typical HPW methods, flat ends of two workpieces are carefully aligned and held under a light initial load to focus heat generation at the interface. As the current pulse is discharged through the workpieces, the intense heat generated at this interface diffuses axially, softening the adjacent material. After a preset delay, an upset force is then applied to produce a forge weld at the interface. Only a few seconds are required from initiation of the pulse to completion of the weld. HPW may be used to rapidly join lengths of pipe in pipeline construction. It is particularly desirable for application in deep water offshore pipeline construction systems. HPW may also be used in joining rails in railroad construction. HPW may also be applicable for brazing, compaction, or forging.
The homopolar generator used in HPW methods is based on the principle that a conductor moving normal to a magnetic field generates an electric potential difference between the conductor ends. In one such type of generator, the conductor is a disk or rotor rotating normal to the direction of a magnetic field generated by field coils surrounding the disk. A potential difference is generated between the rotor outside diameter and the rotor shaft. Sliding contacts on the shaft and rotor serve as current collection devices. A "disk-type" or "drum-type" generator may be used in connection with the present invention.
FIG. 1 shows an exemplary embodiment of a "disk-type" homopolar generator 10 that may be used in connection with the present invention. Further details of homopolar generators are disclosed in U.S. Pat. No. 4,544,874, the disclosure of which is incorporated herein by reference.
Other solid state, forge welding methods differ from HPW in the nature of the energy source, joint preparation, and heating rates used to distribute heat to adjacent material. Such welding techniques include friction welding, flash butt welding, and shielded active gas (SAG) forge welding.
For most electric resistance welding processes, welding heat primarily is generated at the interfaces and diffuses to adjacent material. Except for high frequency resistance welding techniques, intense interface heating results from contact resistance to the welding current flow. High frequency resistance welding and SAG forge welding use a high frequency current, flowing through the joint surface between the electrodes, to generate interface heat. Skin depth effects, controlled by the frequency of the alternating current, constrain heating to the near-surface material.
Assorted current sources, including alternating and direct current types and stored energy machines using capacitor banks, provide the energy for other resistance welding methods. Except for capacitor discharge welding techniques, which have significantly more rapid heating rates than other techniques, heating rates for other resistance welding techniques are considerably slower than for homopolar welding. Friction and inertia welding use frictional heating from the relative motion between workpieces to introduce heat to workpieces. Intense heat generated at the interface diffuses to the adjacent material, heating it to its forging temperature.